This invention relates to semiconductor transducers in general and more particularly to a transducer structure which employs an integral structure formed of highly doped diffused regions. The resultant structure as fabricated by the methods described includes a complete integral semiconductor transducer which possesses great advantages over those transducers depicted in the prior art.
Presently semiconductor transducers, because of their relatively small dimensions, are finding wide use in a variety of applications. These devices, although extremely small, possess high reliability and increased response. Such devices are widely utilized in the medical field, the electronics field, and various other fields for making pressure measurements. It is, of course, understood that for many stringent applications great care has to be taken in providing a small and reliable device while making the device as sensitive as possible. In regard to this, certain applications require that the piezoresistive elements be dielectrically isolated from the diaphragm on which they are located. Other applications require that the entire bridge array, which consists of piezoresistive elements, be hermetically sealed. In certain other applications it is required that there be a stop mechanism to prevent the transducer from being subjected to excessive forces. Hence, the prior art is replete with a number of patents which show various devices adapted to solve these problems. It is understood that devices incorporating these features require different processing techniques which oftentimes are difficult and time consuming to implement.
Essentially, the piezoresistive transducer employs a silicon resistive element, the resistance of which varies according to the intensity or magnitude of an applied force upon an associated diaphragm. Such resistors comprise serpentine or tortuous line patterns. The piezoresistive element, varies resistance according to the intensity of an applied force. The force is usually applied to a relatively thin diaphragm of metal or semiconductor material which is a membrane like structure and to which the semiconductor element is mounted or otherwise diffused therein. A force applied to the diaphragm serves to deflect the diaphragm and hence causes the associated piezoresistive element to vary resistance in accordance with the deflection. The force being measured is transferred through the diaphragm to the strain responsive element causing the element to expand or compress. This produces a change in the resistance of the element. Such elements are conventionally arranged as Wheatstone Bridge circuits with one to four of the bridge legs being active. For examples of typical prior art transducers, reference is made to U.S. Pat. No. 4,063,209 entitled "Integral Transducer Assemblies Employing Built In Pressure Limiting" issued on Dec. 13, 1977 to A. D. Kurtz et al. and assigned to the assignee herein. In that patent there is shown a transducer of an H-shaped cross section which employs a depression relatively equal to the line width of the diffused piezoresistor which is located in the depression. The depression is sealed by means of a glass member which acts as a stop for the transducer for all forces in excess of a rated force which causes a maximum diaphragm deflection relatively equal to the depth of said depression. U.S. Pat. No. 3,930,823 issued on January 6, 1976 to A. D. Kurtz et al. and is entitled "A High Temperature Transducer and Housing Including Fabrication Methods". This patent shows a dielectrically isolated pressure transducer which includes a silicon diaphragm having on a surface at least one piezoresistor sensor mounted in close proximity with a dielectric insulator. The diaphragm is secured about a nonactive peripheral area to an annular ring housing by a glass bond fabricated from a glass material having a low melting temperature when compared to ordinary glass.
As one can see from these patents and various other patents, there is a constant desire to improve the operation of the piezoresistive transducer and to further create a device which is reliable and economical. Thus, according to this invention, there will be described a technique for producing an integral transducer which technique involves the selective diffusion of highly doped semiconductor material to produce the main features of the transducer structure. By utilizing this technique, many great advantages can be obtained over transducers as existing in the prior art, while the resulting transducer further possesses great versatility in regard to accommodating many different modes of operation and structures. The device is simple to construct and provides a very powerful transducer exhibiting improved performance and operation.
The device to be described is fabricated by means of diffusion and selective etching and employs similar techniques as described in a copending application entitled "Fabrication of Dielectrically Isolated Fine Line Transducers and Apparatus", Ser. No. 804,761, filed on Dec. 5, 1985 for A. D. Kurtz et al. and assigned to the assignee herein.
The device to be described provides a raised peripheral flange which is fabricated on a sacrificial wafer and is dielectrically isolated from a piezoresistive pattern of high conductivity semiconductor fabricated from the same sacrificial wafer. The raised peripheral flange enables one to bond a thick sheet of glass over the pattern, thus covering the same. The device gives one the ability to make a hermetically sealed unit with total isolation, while further providing a stop for the unit and dielectrically isolated contact areas. The structures so resulting are then bonded to a carrier wafer to complete the transducer structure.